This invention relates to a security apparatus for use with fencing. In particular this invention relates to an apparatus and method for receiving the longitudinal tension in a taut wire associated with a fence and for producing a tension signal when the longitudinal tension in the taut wire changes by a certain magnitude. Further, this invention encompasses an apparatus and method for monitoring both the tension in the taut wire and electrical resistance of an electrically conductive loop that includes the taut wire.
Fencing by itself provided one of the earliest forms of security for containing animals and limiting access to certain areas. In some cases, however, additional security devices are necessary to detect when a person climbs over a fence, when a fence is cut, or when the fence is otherwise tampered with in an attempt to intrude.
Taut wire fencing systems were developed to detect intrusions into a fenced area. U.S. Pats. Nos. 4,829,287 and 5,103,207 to Kerr et al. and No. 4,367,459 to Amir et al., describe such taut wire systems. In these systems, the taut wire was tightly strung between stationary end posts. The taut wire was then firmly attached to at least one group of pole mounted sensors and loosely attached to other support poles. The pole mounted sensors were adapted to produce an alarm signal when the taut wire moved in a transverse direction. An intruder who was attempting to climb over the taut wire fence would cause the taut wire to move and thereby cause the pole mounted sensor to produce an alarm signal.
With these prior taut wire systems, however, all sensors were pole mounted. The systems were easily circumvented by clamping or otherwise isolating the taught wire to adjacent posts such that the taut wire did not change position relative to the sensor. These prior taut wire security systems were also easily circumvented by cutting the taut wire simultaneously on either side of the sensor.
Additionally, these prior taut wire systems were not useful for all applications. For example, these prior systems could not perform the dual functions of confining livestock and preventing human intrusions because inadvertent contact by livestock would produce false trip signals. Further, when these prior taut wire systems were installed in areas with heavy brush and trees, the systems often gave false signals when wind blown brush fell against the fences.
U.S. Pat. No. 4,318,088 to Hunter describes another type of security fence system in which an electrically insulated single-strand wire was strung in multiple parallel paths around a fenced area to form an electrically closed path. The conductors were then electrified at a low voltage and the current through the conductor was monitored. When the parallel lines of the conductor were pushed together to make electrical contact such as by an intruder, the current in the conductor changed producing a sensor signal to activate an alarm.
However, this conductor monitoring device also suffered shortcomings. The electronics required to monitor the current or voltage on the lines required substantial maintenance. Further, variations in weather could cause false tripping signals. Animals could inadvertently trigger the above described device and therefore the system could not be used by itself to contain livestock. Further, because the system only measured the electrical properties of the line, the conductors could be physically displaced some distance without the system triggering. Also, it was possible to jumper the conductors to bypass the system's fence monitoring function.